1. Field of the Invention
The invention generally relates to braking elements, especially brake drums, and processes for preparing the elements.
2. Background of the Invention
Generally, a braking element comprises two surfaces. One surface is a support surface which acts as an interface for attaching the braking element to a brake assembly via a brake shoe or the like. The other surface is a friction braking surface which is attached to the support surface. The friction braking surface usually provides the braking friction through contact with another surface, e.g., brake plate or brake lining.
The different properties required for each surface usually necessitate that a different type of material be used for each surface. For cast iron brake elements, grey cast iron or "grey iron" and ductile cast iron or "ductile iron", or steel are typically employed. Grey iron is used to prepare friction braking surfaces because of its stable coefficient of friction and excellent wear resistance. U.S. Pat. No. 4,760,900 to Shima et al. Ductile iron or steel is employed to prepare support surfaces because of its ductile strength characteristics. These characteristics allow the ductile iron to withstand the stress and pressures associated with interaction and attachment to the operating elements of an vehicle's braking system assembly.
As a result of using two types of materials, it is conventional practice in the brake art to prepare each surface separately and then bond them together in some fashion to form the braking element. For instance, the '900 patent to Shima et al. discloses metallurgically bonding a friction braking surface of pre-cast grey iron to a high strength metal support surface. As a result, two process steps are employed, i.e., the first step being the preparation of the two surfaces and the second step being attachment of the two surfaces.
In U.S. Pat. No. 3,087,814 to Smiley, a somewhat different two step method is disclosed. Specifically, Smiley discloses applying to a molding surface one layer of metal powder to which is then applied a second layer of powder comprising a "friction" mixture, e.g., iron, graphite and molybdenum. The two layers are then briquetted under pressures in the range of 60,000 to 100,000 pounds per square inch. The resulting briquetting produces coherent self-containing layers which are coextensively related and connected to each other through mechanical bonding. The first layer forms the ductile support layer and the second layer forms the friction braking layer. However, as evident from the description above, not only is a briquetting procedure required, but two processing steps are required in adding the powders necessary to form the braking element.